The primary objective of this study is to test the hypothesis that the age-related decrease in protein synthesis in rat liver is due to changes in mRNA synthesis or in the stability of mRNA. The synthesis of mRNA will be followed by measuring the incorporation of radioactively-labeled orotic acid into poly A-containing mRNA of hepatocytes obtained from rats of various ages. The specific activity of the UTP pool in the hepatocytes will be determined using high pressure liquid chromatography. The specific activity of the UTP pool will be used to calculate the actual rate of mRNA synthesis in hepatocytes. The molecular length and heterogeneity of poly A-segment in mRNA isolated from hepatocytes from rats of various ages will be determined. Changes in the length or heterogeneity of the poly A-segment will be used as an indication of changes in the stability of mRNA.